I am looking for the name of a short story, maybe from the early 1950's. I tried most famous authors without success.

Plot: all humans are extinct after a great war (decades ago). Fighting robots are rusting, wearing out, running down, etc. They try to build new robots, but fail: their giant metal claws cannot assemble the tiny parts of a new "brain". One robot tries to touch a butterfly, but crushes it. One by one they all "die".

1 Answer
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That sounds like "Rust" by Joseph E. Kelleam. You can find this story in Isaac Asimov Presents: The Golden Years of Science Fiction, edited by Asimov and Greenberg and published by Bonanza. The clumsy claws and inability to build new robots are a dead give away.

The protagonist is X-120:

He and his companions were highly developed robots, the last ever made by the Earthmen. X-120 consisted of a globe of metal, eight feet in diameter, mounted upon four many-jointed legs. At the top of this globe was a protuberance like a kaiser's helmet which caught and stored his power from the rays of the sun.

From the "face" of the globe two ghostly quartz eyes bulged. The globe was divided by a heavy band of metal at its middle, and from this band, at each side, extended a long arm ending in a powerful claw. This claw was like the pincers of a lobster and had been built to shear through metal. Four long cables, which served as auxiliary arms, were drawn up like springs against the body.

Later:

But never in all our years have we made a single robot that resembled ourselves.